Refraction or bending of light rays occurs at an optical surface separating optically transmissive media that have different indices of refraction (that is, light travels at a different speed in these media). Numerous refractive surface shapes have been used separately and in combination for focussing parallel light rays to a point or line, or equivalently, for collimating light rays emanating from a point or line source of light. These refractive surfaces are approximate optical forms in which optical performance is traded-off against (a) the cost of fabricating the surface(s), (b) the fabrication tolerances, (c) the number of surfaces used, (d) the tolerance in the location of the surfaces with respect to each other and with respect to the image or focal plane, and (e) the temperature sensitivity.
Refractive forms are particularly useful when used with laser beams. Laser radiation is monochromatic; that is, it (ideally) contains only one wavelength of light. In transmissive optical media, light of different wavelengths travel at different speeds within the medium. Chromatic aberrations occur when light of different wavelengths focus at different points. Because laser radiation is monochromatic, the dispersion of light in transmissive optical media--which is responsible for chromatic aberrations--is essentially nonexistent.
The purpose of a laser beam expander is to expand the width of an incident laser beam to provide a broader laser beam and reduce the laser beam divergence, that is the angular spread of optical rays.
A conventional telescope or beam expander requires at least two lens elements and an intervening optical bench. An objective lens (or lens group) is used to focus the light rays ideally to a single point. A secondary lens (or lens group), sometimes called an eyepiece, is used to recollimate the light with some amount of magnification or demagnification which depends on the relative focal lengths of the objective and eyepiece. For most telescopes, a tubular housing serves as the optical bench.
Telescopes or laser beam expanders are of two types: reflective and refractive. All reflective telescopes or beam expanders require an optical bench to maintain alignment between two reflective optical surfaces and suffer from the problem that one of the surfaces always obscures the other when operated on-aperture with the incident beam.
In the reflective optical telescope forms used in modern astronomical telescopes the secondary mirror, which is centered on the optic axis, obscures the central part of the aperture of the primary mirror (primary aperture). The diffraction effects of the central obscuration and problems associated with suspending the secondary mirror within the primary aperture can be eliminated by offsetting the primary aperture to a region of the primary mirror that is not obscured by the secondary, with a corresponding decrease in off-axis performance. This is an off-aperture optical system. A refractive telescope has no central obscuration because the secondary lens does not block the primary; therefore, there is no need to transmit light off-aperture.
A paraboloid reflective optical surface is an exact optical form. For a paraboloid surface defined by rotating a parabolic curve about its axis of symmetry(optic axis), all rays parallel to the optic axis are focussed perfectly to a single point on the optic axis which is the mathematical focus of the paraboloid curve. Two such paraboloid surfaces of revolution can be arranged in a confocal configuration in which the optic axis of the surfaces are identical and the mathematical foci of the two parabolic shapes are coincident on the optic axis.
A similarly defined confocal paraboloid telescope or beam expander optical form is also an exact form. All rays parallel to the optic axis are focussed by the first surface and perfectly recollimated by the second surface such that the optical path length difference between these rays is precisely zero and they emerge from the second surface parallel to one another.
Numerous refractive systems have been used for magnifying images or expanding optical beams, most employing at least two air-spaced optical elements held in alignment by an optical bench.
It would be desirable to remove the disadvantages of the present reflective and refractive beam expanders and telescopes. If, further, this task could be achieved with a reduction in parts, the benefit to the art of laser beam expansion would be unparalleled.
Another issue concerning laser beams involves the problems attendant in laser diode arrays. A laser diode array consists of an array of individual diode bars, where each diode bar serves as a source of light. All of the individual diode bars together serve as an array of individual light sources. Each individual diode bar is composed of linear arrays of light emitting p-n junction diodes that are configured such that the emitting region is within one of the layers of the planar semiconductor diode structure. The edges are cleaved and coated to form an optical resonator and lasing occurs in a direction normal to these edges, hence the descriptor known in the art: "edge-emitting" diode. The active region of each laser diode is bounded by the cleaved surfaces in the longitudinal direction and by lithographically defined semiconductor structures (grooves, implanted regions, etc.) in the transverse direction. Usually hundreds of these laser diodes are included on a single substrate or "bar". The diodes are arranged side-by-side and all share the same output edge. The laser light emitted from the output edge of such an edge-emitting diode bar is reasonably well collimated in the direction corresponding to the long dimension of the array due to the constructive interference between the "in-phase" emitters (similar to a radar phase array antenna). The laser light is not well collimated in the direction corresponding to the narrow dimension, since the emitting aperture is very small and diffraction causes the light to spread considerably. For most applications, the laser light from the diode must be collimated in this direction with a cylindrical or anamorphic lens. For a laser diode pumping application, the task is to relay the light from the diode bar to a laser rod, which generally requires collimation and then focussing.
To create a two-dimensional array of laser diodes, whether conventional or monolithic, it is necessary to stack multiple edge-emitting diode bars. This makes cooling of the diode bars difficult because the diode bars are most efficiently cooled through their flat surfaces, and these surfaces are more difficult to access in a two-dimensional structure. Conventional approaches mount the individual diode bars on thin coolers which can be stacked together to form a two-dimensional (2-D) array, the so-called "rack-and-stack" configuration. Some structures used today employ a monolithic shelf configuration on which the diodes are placed and these structures cool the entire shelf through the back surface of the diode bars. The shelf approach is limited by the low thermoconduction through the shelf itself since it relies on conduction alone for the thermal transfer.
Conventional cylindrical optical lenses, available from several vendors, are now used to collimate the output of conventional "rack-and-stack" as well as shelved arrays. These lenses are mounted in close proximity to the emitting edge of the bar and are arranged with the cylindrical edge along the long dimension of the array. A perfect cylindrical lens would act to preserve beam quality of the laser in the narrow dimension; that is, the product of the beam divergence and the emitting aperture measured out of the diode would be the same as the divergence-aperture product measured at the output of the collimating lens. Practical optics can never perfectly conserve beam quality due to: a) aberrations caused by the optical prescription itself, b) the construction of the lens, c) placement of the lens with respect to the diode bar, and d) warpage of the diode bar.
The aberrations associated with a conventional refractive optic become more severe as the optic is forced to operate off-aperture from the light-emitting axis of the diode bar; additional optical elements can be retired to adequately correct the aberrations.
It is desirable to eliminate the problems of a) conventional reflective and refractive laser beam expanders, as well as the problems of (b) conventional 1 dimensional and 2 dimensional laser diode arrays.